The girls, whose mother is a poor, widowed domestic servant, were last seen outside a cheap roadside café selling rice and dahl, and may have decided to follow their killer because they were hungry.

The case has provoked fresh outrage in India which has seen a sharp increase in rape and sexual violence cases reported since the gang-rape and murder of a Delhi student on a moving bus in December.

Villagers in Maharashtra state protested over the failure of the police to act after the girls' grandfather reported them missing last week.

When the police found the three girls' bodies on Saturday in a well near Murmadi, two miles from their home in Lakhni village, they initially recorded their deaths as "accidental".

An autopsy later confirmed the girls, aged six, nine and eleven, had all been sexually assaulted, while the cause of death has not yet been established. There were no signs of external injuries, police said.

The state's chief minister pledged £10,000 in compensation while an Indian cabinet minister said he was "pained" by the murders.

"We have rounded up a few people for questions and investigations are on.

We have few leads and we are working on them. There was delay because the mother was in a state of shock, she didn't speak at all, we are trying to join the dots from her statement," said Superintendent Aarti Singh.

"We have suspended a police officer and started an investigation against him for dereliction of duty. We are at presently focusing more on finding the perpetrators but if anybody from the police force is found guilty, he will be dealt with strictly," she added.

Praful Patel, a local MP and industry minister, said the culprits should be "hanged for this".

"The survivors should be given speedy justice," he said.

Child rights campaigners said rape and killing of the three sisters highlighted India's urgent need for a credible child protection system.

It is often children, particularly girls with single mothers who are exploited, abused and made to suffer in the deeply patriarchal set-up in India," said Thomas Chandy, chief executive, Save the Children India. The lack of a strong child protection mechanism exacerbates the problem."